I tried to model a realistic solar system in Blender 2.48a with correct sizes in diamater of the planets and their rotations around the sun, but the clip "end end" in the "view properties panel" goes "only" up to 1.000.000.

The world in Blender seems to be limitet to 1 Mio units, is that right ?

Is there a possibility to increase this value (to say 100 Mio or more) ?

If i resize everything to fit the world, the size of the planets become inacurate because the lowest value is "only" 0.001.

Is there a workaround to increase this limited "worldresolution" or another possibility to achieve a greater range of values than from 0.001 to 10000 or 1:1 Mio.

The problem is more complicated than you realize and just changing a magic number deep in the core of Blender will not fix it.

The z-buffer (aka the depth buffer), which is the thingie used to decide what is in front of what when rendering, has a fixed number of bits. These bits are spread out between clipStart and clipEnd with greater resolution nearer clipStart. This is also nearer the camera where more interesting things are assumed to be happening.

By spreading the z-buffer bits over a greater range - the effect of increasing clipEnd - you get rendering artifacts where pixels do not get drawn at their proper depth.

Your best bet is to scale your model to fit and make the difference between clipStart and clipEnd as small as possible. Making clipStart as near the scene as possible also helps. Remeber that computer graphics is all about fakery.